In the past, when it was desired to determine the "health" of a gas turbine engine, various engine operating parameters would be measured and recorded during a test flight by a maintenance crew. The recorded data would subsequently be employed to determine the health of the engine and, by way of example, whether turbine blade wear had resulted in a decrease in the maximum output power which could be produced. Such a measurement procedure is time consuming and expensive. Furthermore, knowledge that the maximum output power available from an engine had decreased by a small percentage did not help the pilot in determining the maximum power available under a specific anticipated flight condition.
Engine performance "maps", either supplied by the engine manufacturer or derived from data supplied by the manufacturer, could theoretically be used to predict the engine power available under varing operating conditions. Such predictions, however would not be sufficiently accurate since they would be predicated upon baseline curves which are themselves not sufficiently accurate over the full flight envelope. Further, since engine performance "maps" do not take into account reduced performance resulting from wear, they are of value only to maintenance crews.
The safety of operation of a gas turbine engine powered aircraft, and this is particularly true of rotary wing type aircraft, would be significantly enhanced if the pilot could be provided with information which would allow him to know, in advance of attempting to operate under a given set of flight conditions, the "health" of his engine and particularly the power available from the engine. This same information would be helpful in determining the most desirable time for performance of routine engine maintenance.